How long has this functionality been around? The marksheet tab mystery unravelled!
I have never had classes assigned to me until today when I became a "form tutor" (long story). This has had the benefit of exposing some functionality that, as an administrator, has been hidden from me for about five years and solves the riddle of why not all marksheets appear in the register tab strip!
Go to a class, right click the marksheet tabs at the top of the page and you can configure the tab strip to pick which five sheets you want to see and hide My list Entry!
This doesn't work as an admin as you only see the tabs if you are the class teacher and thus cannot configure them.